CONCENTRATION FLUCTUATION ENHANCEMENT IN POLYMER-SOLUTIONS BY EXTENSIONAL FLOW

When subjected to flow, various polymer solutions undergo an 'apparent '' change of phase, which is manifested by a dramatic increase in the turbidity. This phenomenon is the result of anisotropic flow-induced g rowth of concentration fluctuations, which can be predicted qualitativ ely by the coupled equations of motion for the concentration and veloc ity fields. We have investigated the growth of concentration fluctuati ons of poor and near-THETA semidilute solutions of polystyrene in dioc tyl phthalate (PS/DOP) subject to plane extensional flow at temperatur es above the quiescent cloud point. Scattering dichroism and small-ang le light scattering (SALS) indicate that concentration fluctuations gr ow perpendicular to the principal axis of extension for low strain rat es. For higher extensional rates, fourfold symmetry appears in the str ucture factor, with intensity maxima on the axes at 45-degrees to the principal axes. This fourfold symmetry is predicted by a recent theore tical model. We also show that the strength of flow-induced scattering is linear to first order in the ratio of viscoelastic stress to osmot ic pressure.